This invention relates to a method of communicating digital signals and a receiver for use with such method particularly, but not exclusively, applicable to the communication of digital video signals via a passive optical network.
CATV systems have, to date, used analog transmission schemes because the cost of digital equipment and the bandwidth required for transmission is high but use of digital techniques in television receiver design is increasing with a/d conversion at the input to a single digital processing integrated circuit. Digital coders are now becoming available which have the potential for low-cost implementation and which provide a picture quality that compares favorably with off-air reception. Among the benefits of digital transmission to users are guaranteed picture quality, the possibility of adding new services to receivers equipped with more advanced digital signal processing, the elimination of the requirement for a/d conversion. Until now the extensive use of digital transmission has been limited by bandwidth and cost constraints of the copper network. However new, low-cost broadcast optical networks are emerging which allow a mixture of services to be provided by time domain or wavelength domain multiplexing (TDM or WDM) on an evolutionary basis. The speed limitation of the electronic components connected to the network rather than the network itself is now the limiting factor on the digital channel capacity per wavelength. Recent papers entitled `Single mode optical networks` by Payne D. B. and Stern J. R. Proc. Globecom'85 and `Technical Options for Single Mode Local Loops` TDM or WDM by Payne D. B. and Stern J. R. Proceedings of ECOC 1986, Barcelona have discussed the merits of multiple access local networks based upon passive optical networks which use power dividers to serve a large number of customers. These include the possibility of broadband transmission on a single carrier and upgrading via wave division multiplexing (WDM). Whilst WDM would provide one solution to this problem, TDM seems more attractive because only a single transmitter is required.
The passive, multiple-access architecture has a number of operational advantages as discussed in the paper on single mode optical networks referenced above. The need for cables with large number of fibers near the head end is reduced, it is more reliable and easier to maintain than a network with active switching at splitting nodes, and it is possible to upgrade the network as wavelength multiplexing components become available. These networks are most cost sensitive in the customers' final drop and equipment as the cost of the head-end is shared by all users. An objective of the present invention is to provide a method and a receiver for use with such a method whereby customers can access a large number of digital TV channels in a way which can reduce the cost of customer equipment by eliminating the need for digital circuitry operating at the multiplex rate.
It is a further object of the present invention to provide a method of signal communication and a receiver in which a level of security can be given to some or all of the multiplexed channels by scrambling prior to transmission and in which the channel selection and descrambling is carried out automatically by the receiver.
According to one aspect of the present invention there is provided a receiver for selectively extracting one of two or more time division multiplexed channels of digital signals, which channel has been scrambled before forming the multiplexed signal using a predetermined digital sequence comprises
a clock means arranged to provide clock pulses having a repetition rate dependent on a clock control signal; PA1 a digital sequence generator means for providing a generator signal representative of the next digit in the predetermined digital sequence at the clock repetition rate; PA1 a sampling means arranged to provide a sample signal which is the multiplexed signal sampled at the clock pulse rate; and PA1 a phase lock means controlling the clock control signal whereby the generator sequence is phase-lockable to the sampled signal.
The scrambling and descrambling is preferably by XOR-ing with the scrambling and descrambling sequences as this tends to produce a balanced data signal. The delay lock means is arranged such that when there is no correlation between the sampled and generator signals the clock control signal produced causes the clock pulse repetition rate to be slightly different from (it can be above or below) the channel bit rate. There will therefore be phase slip between the sampled signal and the generator signal. When these streams coincide, a descrambled video channel is output. Additional circuitry is required to detect the presence of this video channel, so that the delay-lock loop may be closed. One method of achieving this is as follows.
The video channel contains long strings of zeros in the line blanking intervals, and these are detected using an 8-bit ECL serial/parallel converter with wired `or` outputs. This produces a logical `0` output during the line blanking interval if the channel was scrambled prior to transmission by a sequence corresponding to the receiver's descrambling sequence. The descrambled signal is fed to a circuit designed to lengthen the zero level pulses. The change in signal level which occurs when this string of descrambled zeros is detected is integrated, altering the frequency of the VCXO to give delay-lock at the incoming channel rate.
Alternatively the delay lock means may comprise a comparison means providing an output signal representative of the XOR of the generator signal and the sampled signal and an averaging means arranged to provide the clock control signal such that the clock control signal is representative of the time-averaged d.c. component of the output signal. It will be clear that the above invention can be operated with logical ls as well as logical Os as the line blanking signal by the inclusion of inverters in the appropriate places.
Delay-lock occurs at the edge of the appropriate channel `eye` pattern in the multiplex, where incoming data transitions provided a suitable reference point so the descrambled data in the delay-lock loop therefore is prone to errors. To achieve error-free operation of the video channel, it is preferred that the input data is also sampled by a second D-type bistable, clocked at the centre of the `eye` pattern via a T/2 clock delay line. This data is then descrambled independently using the locally generated PRBS.
Using this method of communication the receiver will automatically select a channel which has been scrambled by the same sequence as generated by the sequence generator in the receiver and at the same time automatically adjust the phase of the clock relative to that channel thereby obviating the need for a clock recovery system independent of the descrambler.
The demultiplexer selects a single channel using a sampler clocked at the channel rate with a phase determined by the delay-lock loop. A benefit of this method of demultiplexing is that clock recovery and time slot synchronization is achieved at the baseband rate enabling all the receiver electronics to operate at the channel rate. A further benefit is that the same channel selection technique operates independently of the number of channels up to the speed limitation of the sampler.
Conveniently, the digital sequence is provided by a dedicated pseudo-random sequence generator although alternative methods can be employed for example reading out a sequence stored in an EPROM or generating the sequence by a general purpose computer.
Preferably the pseudo-random sequence generator is a stream cipher.
According to another aspect of the present invention a method of communication is provided in which a transmitter transmits two or more time division multiplexed digital signals, one or more of which has been scrambled using a predetermined digital sequence, and selectively extracting the scrambled signal by means of a receiver according to the present invention.
According to a yet further aspect of the present invention a method of transmitting a digital signal is provided in which the digital signal is scrambled using a predetermined digital sequence and time division multiplexed with one or more other digital signals and the multiplexed signals transmitted to one or more receivers.
Various service protocols between the transmitter and the receivers to which the multiplexed channels are transmitted can be set up. In one arrangement the receiver is provided with means whereby the customer can alter the sequence generated by the sequence generator. This can be readily achieved by providing a pseudo-random sequence generator (PRSG) whose output is determined by a characteristic function which can be set by, for example switches, in known manner. The customer can then select one or more of the channels to be received by setting the characteristic function to that of the desired channel. By providing the customer with the means to set only a subset of the characteristic functions of the channels at his receiver he will be free to select a channel from that subset of channels only. The provider of the signals to the customer can alter the availability of channels to such a customer by choosing which channels are scrambled with the sequences available a particular customer. Each customer may have a fixed, unalterable set of available characteristic functions distinct from another customer yet they can receive the same data if the data is separately scrambled onto two separate channels using two different scrambling sequences.